A thrust reverser of the so called target type is, for instance, illustrated in U.S. Pat. No. 4,966,327. Thrust reversers of this kind include a pair of half-shells hinged to the wall of the engine exhaust nozzle. These shells are movable from a retracted position in which they form a portion of the exhaust nozzle to a deployed position where they form a wall which reverses the jet output by the nozzle. The thrust reverser includes actuators which are responsible for the rotation of the half-shells between their retracted and deployed positions.
A thrust reverser of this known type is illustrated in FIGS. 1, 1a, and 2 in the drawings attached. FIGS. 1 and 2 are schematic side elevations showing the half-shells in their retracted and deployed positions respectively, while FIG. 1a shows the device with retracted shells in a perspective view. With reference to these figures, the exhaust nozzle 1 of an aircraft jet engine can be terminated with a substantially round edge 2. Nozzle 1 extends beyond edge 2 with two diametrally opposite lugs 3, hinged at 4 to which there are two movable half-shells 5 which can be set in a retracted position, shown in FIGS. 1, 1a where they form the last part of the exhaust nozzle, and a deployed position, shown in FIG. 2, where they form the wall which deflects forward the jet output by the engine to obtain the thrust inversion. The movement of the shells is caused by two hydraulic cylinders 6, each held by lug 3, and each inclusive of a piston, not shown in the drawing, connected to two elements 7 which emerge from the cylinder body through two slits in its sides. Elements 7 are connected to actuating rods 8 which are hinged to the two shells 5. As shown by FIG. 2, the curved edges 5a of the two half shells 5 enters into contact in the deployed position of the shells. Due to this, when the shells are retracted, they define an exhaust portion of the exhaust nozzle, with a "fish mouth" cross section.
As the edge 5a of each shell has a variable thickness, in particular is thicker in the middle and thinner at the edges, such "fish mouth" profile gives way to a pair of steps 5b (see FIG. 1a) close to the edge of the exhaust nozzle 1, which cause a degradation of the aerodynamic characteristics of the exhaust duct of the engine during normal flight, with a consequent loss of performance.
The geometry of the exhaust duct of a jet aircraft engine is in fact defined by the manufacturer so as to optimize performance in terms of thrust and of specific fuel consumption at the different operating rates. The geometry selected by the manufacturer corresponds therefore to the best compromise between the various geometries in the different flight conditions. A modification to the exhaust duct caused by the adoption of a thrust reverser of the type described above, will therefore necessarily cause an overall loss of performance throughout the flight profile of the aircraft except at touch-down and also, in the specified case above, an appreciable loss of the thrust coefficient CT and of discharge coefficient CD compared to an ideal nozzle, with a consequent reduction of the available thrust and/or increase of specific fuel consumption for equal engine rates. Steps 5b described above also cause a base drag which increases the drag at null aircraft lift.